1. Field of the Invention
The invention relates to a method for determining the compression in the cylinder of a supercharged, directly injected internal combustion engine with a high-pressure reservoir, from which fuel is delivered to an injector, and a supercharger for increasing the air throughput by compressing the aspirated air for the engine.
Internal combustion engines with direct injection are very promising with regard to reducing fuel consumption with relatively low pollutant emissions. In contrast to intake tube injection, in direct injection fuel is injected at high pressure directly into the combustion chamber.
Injection systems with a central pressure reservoir (common rail) are known in the prior art. In those so-called common rail systems, a fuel pressure regulated by an electronic control unit of the engine, via pressure sensors and pressure regulators, is built up in the common rail and is available largely independently of the engine speed (rpm) and the injection quantity. The fuel is injected into the combustion chamber via an electrically driven injector. The injector receives its signals from the control unit.
By functionally separating the pressure generation and the injection, the injection pressure can be selected largely freely, regardless of the current operating point of the engine.
Modern electronically controlled injection systems for diesel engines are time-controlled, as compared with the old mechanically controlled systems. In the latter, the reference point is the compressed volume and they moreover have a fixed relationship between the feed rate and the engine speed, and hence a fixed relationship between the injection pressure and the speed. This means that in conventional systems, with the same adjustment of the quantity adjusting mechanism, the same fuel volume is always injected regardless of the ambient conditions prevailing at the time. By comparison, in time-controlled systems, a triggering time must be ascertained on the basis of a calculated injection mass.
In common rail systems, in which the injection pressure can accordingly be adjusted independently of the operating point of the engine, the injector triggering time required for a particular injection mass is dependent, under constant ambient conditions, on the pressure drop prevailing just at that time at the injector, i.e. the injection nozzle. This pressure drop is the difference between the rail pressure p.sub.rail in the high-pressure reservoir and the pressure in the combustion chamber, that is, the cylinder pressure p.sub.cyl.
To calculate the triggering time for the injector, the pressure drop .DELTA.p=p.sub.rail -p.sub.cyl at the injector must accordingly be taken into account. To that end, the rail pressure and the compression in the cylinder must be known, if at all possible at the instant of injection onset.
The rail pressure is measured with a pressure sensor and made available to the electronic control unit of the engine for further processing. Conversely, typically there is no sensor present for measuring the compression, so that the pressure in the cylinder has to be calculated on the basis of known variables.